1. Field of the Invention
This invention provides a method of testing the integrity of an ultrasonic system for sensing liquid-fluid interfaces.
2. State of the Art
According to the prior art, a typical ultrasonic system for sensing a liquid level, or more generally for sensing a liquid-fluid interface, comprises means for continuously transmitting an ultrasonic signal of at least 500 kilohertz, means for receiving the transmitted ultrasonic signal, and means for detecting the received signal. Typically, the detecting means is connected to means for operating a valve, pump or servomechanism.
In a typical liquid-fluid interface sensing system of the prior art, a transmitting transducer and a receiving transducer are disposed to provide a straight-line ultrasonic signal path that intersects the liquid-fluid interface at an angle of incidence of from 0.degree. to 90.degree.. The transmitting and receiving transducers comprise piezoelectric crystals that are interchangeably or mutually positioned about (e.g., above or below) a desired liquid interface level in a detection area. Usually, one crystal is positioned above the desired interface level, and the other crystal is positioned below the desired interface level. However, in particular embodiments, both crystals may be positioned on the same side of the interface.
It is convenient to discuss the prior art and also the present invention in terms of a liquid-gas (or more usually, a liquid-air) interface. However, the "fluid" of the generalized liquid-fluid interface could also be a liquid. Thus, the interface to be detected could be the interface between two different liquids such as water and oil, or a slurry and water.
In operation, an electronic control unit generates an electrical signal, which is converted to an ultrasonic signal at the transmitting transducer. As long as the liquid-fluid interface stays within the gap between the transmitting and receiving transducer crystals, at least a portion of the path traversed by the ultrasonic signal in crossing the gap is through the liquid. After passing through the liquid, the ultrasonic signal is reconverted to an electrical signal at the receiving transducer. The reconverted electrical signal is then amplified to produce an output signal that can be used, e.g., to energize a relay for controlling a servomechanism. However, when the liquid-fluid interface falls below the gap between the transducers, the ultrasonic signal crossing the gap becomes so attenuated that the relay cannot be energized.
In a typical liquid-fluid interface sensing system of the prior art, the transmitting and receiving crystals are supported in a container or receptacle for the liquid to be detected, and associated electronic circuitry is adjusted so that when the level of the liquid to be detected (or more generally, the liquid-fluid interface) reaches the level of the gap between the crystals, ultrasonic transmission through the gap occurs at sufficient strength to maintain self-sustained oscillations in a closed electronic/ultrasonic feedback loop. In the absence of detectable liquid in the gap, however, the system remains in a quiescent non-oscillating state.
In many applications, it is desirable to be able to check the integrity of an interface sensing system of the above-described type before the level of the liquid to be detected reaches the level of the gap between the transmitting and receiving transducers. However, the output signal generated by such a system is typically the same (i.e., a NULL signal) when the level of the liquid to be detected moves away from the gap as when a defect has occurred in a portion of the electronics or in a transducer, or when a transducer cable has been cut or disconnected. Thus, until the present invention it has not generally been possible to test the integrity of an ultrasonic liquid-fluid interface sensing system, where the system is in place for sensing a liquid-fluid interface that has not yet reached the level of the gap between the transducers.
Attempts have been made in the prior art to provide system integrity checks for ultrasonic liquid level detectors. For example, U.S. Pat. No. 3,851,333 discloses an ultrasonic liquid level detector that utilizes a spray adaptor and pump means for creating an environment that simulates operational conditions. However, until the present invention, in-place testing of the integrity of an ultrasonic liquid-fluid interface sensing system such as that described in U.S. Pat. No. 3,520,186 has not been possible.